In cutting a material, a cutting tool and a cutting method suitable for a work material are selected. With regard to achieving a long life during cutting, how a temperature at a cutting edge is suppressed during cutting is important, and a tool material having excellent thermal conductivity is valued. In general, also during cutting using a tool made of an ultra-high pressure sintered body such as a diamond sintered body and a cubic boron nitride (which may also be denoted as “cBN”) sintered body excellent in thermal conductivity, diffusion into a work material or chemical wear such as oxidation develops due to increase in temperature at the cutting edge under such a high-efficiency condition as a high-speed condition or a large cutting-depth or high-feed condition. As measures for suppressing such wear, change to a low-speed condition, suppression of resistance during cutting by decreasing a wedge angle at the cutting edge of the tool, cooling of a cutting point by injecting a coolant toward the cutting point, or the like has been carried out.
For example, as measures for achieving a further longer life during cutting of a difficult-to-machine material, Japanese Patent Laying-Open No. 2009-045715 (PTL 1) uses a cutting tool, in which an ultra-high pressure sintered body material having such a high heat radiation property as thermal conductivity not lower than 100 W/m·K is applied to a portion of a cutting edge at least involved with cutting. Then, this publication discloses an invention for suppressing increase in temperature at a cutting edge due to heat generated by cutting, by carrying out working while the cutting edge of such a cutting tool is cooled with a high-pressure coolant.
Meanwhile, for example, during cutting of a brittle difficult-to-machine material such as glass, ceramics, cemented carbide, or an iron-based sintered alloy difficult-to-machine material, it has been proposed to achieve a good worked surface by softening a work material or varying a mechanism of generation of chips from a brittleness mode to a ductility mode by carrying out cutting under a high-speed condition or by increasing a temperature at a point of cutting of the work material with laser assistance.
In principle, however, the cutting edge of the tool is exposed to a high temperature and also to rapid cooling, and hence a cutting tool tends to degrade and chipping or sudden chipping thereof is likely. In addition, in a machine tool as well, such problems as restriction on the number of revolutions of a main shaft or requirement for installation of an expensive laser apparatus arise.
A cBN sintered body mainly refers to a body obtained by bonding cBN particles to one another with a binder phase mainly composed of TiN, TiC, Co, and Al. The cBN particles are a material having hardness and thermal conductivity next to diamond and being superior in toughness to a ceramics material. Therefore, a cBN sintered body having such a high cBN content that it contains cBN particles by 80 volume % or more is excellent in such characteristics as resistance to plastic deformation, chipping resistance, and the like.
A tool made of a cBN sintered body, which includes the cBN sintered body having such characteristics, is excellent in that it is better in chemical stability, lower in affinity with iron, longer in life, and higher in efficiency in working because of its high hardness as a material, than a tool material such as a conventional superhard tool and the like, and it is highly evaluated. Such a tool made of a cBN sintered body of high performance has replaced a conventionally used tool in such applications as cutting of Ni-based and iron-based high-hardness difficult-to-machine materials, applications of plastic working of a punching tool for cold forging, and the like.
Here, cutting refers to machining of an article having desired dimension and shape while a work material is locally sheared and crushed and chips are generated. On the other hand, plastic working refers to application of force to a workpiece to deform the same and formation of the workpiece into a product having prescribed shape and dimension. It is noted that plastic working is different from cutting in that no chips are generated.
Since the tool made of the cBN sintered body has excellent characteristics as described above, it is advantageous in that sudden chipping is less likely in any application of cutting and plastic working and it is extremely suitably employed.
For example, with regard to a conventional tool made of a cBN sintered body, Japanese Patent Laying-Open No. 07-291732 (PTL 2) and Japanese Patent Laying-Open No. 10-158065 (PTL 3) regard such a metal as Al, oxygen, and the like contained in a cBN sintered body as an impurity. Then, these publications each disclose a technique for improving hardness and toughness of a cBN sintered body by minimizing introduction of such an impurity and increasing a ratio of cBN particles to be mixed.
In addition, a tool made of a cBN sintered body has been considered and commonly believed to be high in performance if it has high hardness and high toughness as well as high thermal conductivity. In accordance with this common belief, Japanese Patent Laying-Open No. 2005-187260 (PTL 4) and WO2005/066381 (PTL 5) each have proposed use of a cBN sintered body containing high-purity cBN particles at high concentration by making use of high thermal conductivity of high-purity cBN particles. Thus, a tool made of a cBN sintered body which achieves improved hardness and toughness as well as improved thermal conductivity can be obtained. Chipping of such a tool made of a cBN sintered body is less likely even in a case of plastic working of a material of low ductility, in particular in a case of cutting of an iron-based sintered alloy, and the tool is excellent also in wear resistance, whereby the tool is suitably used.